For this particular mkv, no subtitles. However, MKVs that have subtitles do play on my xbox (I need to try on my echo but I would assume the same/similar results) - of course the subtitles don't actually display/work (extender limitation), but the mkv containing them will still play. The trick I've had to use was to ensure header compression on the subititles is set to "none" before remuxing, though I swear I've not done that and it still plays... I just always do it now.
Yes, most of my library is full bitrate with HD audio as well. As mentioned already, this is simply an example of an mkv w/DTS that plays 2ch audio since folks were asking - not a standard in any way. This is simply an older rip that I haven't messed with in a long time since it plays fine on everything and my daughter has access to it w/out fuss (most important! )